Typically, a lamp comprises a cap, housing and light source, where at least a part of the light source is accommodated in the housing, the lamp is used by mounting of the cap on a socket, and common modes of connecting the cap and housing include insert-pull out, insert-rotate, or rotate. Upon mounting the lamp on the socket, the positions of the lamp and socket are fixed with respect to each other, and the lamp cannot undergo rotational or telescopic movement with respect to the socket, thereby causing considerable inconvenience. For example, upon mounting a directional LED lamp on the socket, because the lamp is unable to rotate relative to the socket, it is sometimes impossible to adjust the LED light source to illuminate the desired direction. Furthermore, lamps come in a vast array of shapes and sizes, and as the lamp cannot extend or contract relative to the socket, in many cases where the lamp is installed in the fitting, the center of illumination may not match the optical center of the fitting, thereby resulting in the issue of uneven illumination.